The present invention relates generally to the control of rolling mills, and particularly to a method of reducing cyclic effects on the gauge of material rolled in a mill caused by eccentric roll assemblies of the mill while simultaneously and transiently controlling and offsetting the effects of incoming variations in thickness and/or hardness of material on the gauge of the material leaving the mill.
Variations in the thickness (gauge) of material leaving a rolling mill are caused in part by errors in measuring the gauge, incoming gauge variations from previous rolling operations, variations in the hardness of the material to be rolled, and deficiencies in the control system of the mill, which include both the electrical and mechanical components of the system. For example, if one or more of the roll assemblies of the mill has an eccentric characteristic, due to say an eccentric bearing problem, or to a roll that is out-of-round, the eccentric characteristic is imprinted upon the material leaving the mill in the form of a cyclic variation in the gauge of the material. The period of this cycle variation in gauge is that of the circumference of the eccentric roll assembly.
In U.S. Pat. No. 3,709,009 to Shiozaki et al, a system is disclosed in which rolling pressure is sampled for the purpose of calculating roll eccentricity and the phase angle of the eccentricity, using Fourier series of a function. These calculations are then employed to correct for such eccentricity by adjusting rolling pressure in response to the eccentricity such that the material reduced in thickness in the mill is substantially free of cyclic variations in rolling force, which has an effect on exit gauge.
Similar concepts and techniques are employed in U.S. Pat. No. 2,950,435 to Locher et al and in U.S. Pat. Nos. 3,242,341 and 3,496,344 to Chope.
In none of the above patents, however, is there an attempt to transiently control the mill in terms of both periodic and non-periodic variations in the material rolled. In the Shiozaki et al patent, for example, reference is made to the standard gaugemeter equation, but the system employed uses an equation in which the pressure or force .DELTA.P at which the material is rolled is solved to make corrections for roll eccentricity. This is accomplished by use of a combination of roll force and actuator position measurements, as opposed to an actual gauge measurement, since such a combination is an available indication that is instantaneously related to the gauge of material exiting the bite of the rolls. Actual gauge measurement, in the art, is made by a thickness measuring instrument located some distance from the roll bite. There is thus a delay or transport time between the occurrence of a gauge change and the time it is detected by the instrument.
Instantaneous indications of rolling pressure or force (.DELTA.F) can be employed to calculate instantaneous exit gauge by use of the well-known gaugemeter equation EQU .DELTA.h=(.DELTA.F/M) =.DELTA.S
where
.DELTA.h is a change in exit gauge, PA1 .DELTA.F is a change in total rolling force or pressure, PA1 .DELTA.S is a change in the position of the screws or cylinders of the mill, and PA1 M (or K) is the modulus of elasticity of the rolling mill. The fraction .DELTA.F/M or .DELTA.F/K is a measure of the "stretch" of the housing of the stand of the rolling mill and the compression of the roll assemblies of the stand in the process of reducing the thickness of material directed through the stand.
However, the gaugemeter equation above, and the system employed by Shiozaki et al, for example, have no means to distinguish between "in stack" eccentricity problems and variations in the thickness and/or hardness of the material entering the mill or to employ the two variations to reproduce a total transient load variation in a predictive manner so as to eliminate their combined effect on exit gauge.